An access PON is a passive optical network having a point-to-multipoint (P2MP) structure, and includes an optical line terminal (OLT), an optical distribution network (ODN), and at least one optical network unit (ONU) or optical network terminal (ONT). The ONT may be considered as a special ONU, and therefore the ONU is uniformly used in the following in this document.
In an uplink direction (a direction from an ONU to an OLT), all ONUs share an optical transmission medium in a Time Division Multiple Access (TDMA) manner. In the TDMA manner, the OLT performs a bandwidth grant on an ONU. For the ONU, this bandwidth grant is a light emission timeslot of the ONU. Each ONU has its own specific light emission timeslot. The ONU sends an optical signal to the OLT according to the light emission timeslot assigned by the OLT to the ONU. In a downlink direction (a direction from the OLT to an ONU), the OLT sends an optical signal to each ONU in a broadcast manner.
In a PON system, dynamic bandwidth assignment (DBA) is a mechanism in which dynamic assignment of uplink bandwidth to an ONU can be completed in a microsecond-level or millisecond-level time interval. A transmission container (T-CONT) is a basis for implementing the DBA in the PON system, and is a buffer unit for bearing a service. In practice, uplink data requires different DBA templates to be configured according to different requirements of a service, and the DBA templates to be bound to the T-CONT. Four types of bandwidth are defined in the PON, to implement five types of T-CONTs. After a specific T-CONT is defined, a defined service may be mapped to the required T-CONT. The different types of T-CONTs have different bandwidth assignment manners, and can meet different requirements of different service flows on a time delay, jitter, a packet loss rate, and the like.
Currently, the OLT receives, from each ONU in advance, a bandwidth request for an amount of data that is to be sent in the uplink direction, determines, in response to the request, uplink bandwidth that is to be assigned to a T-CONT of each ONU, and provides, to the ONU, a grant notification for sending allowable bandwidth. In the prior art, the OLT sets a maximum bandwidth value for the T-CONT of each ONU within a grant bandwidth period. Therefore, when grant traffic requested by the ONU exceeds the maximum bandwidth value that is set by the OLT, transmission of non-grant traffic is delayed, the non-grant traffic is transmitted in a subsequent DBA period, and the non-grant traffic is temporarily buffered in the ONU. Consequently, when the ONU has massive burst data traffic in the uplink direction, a response from the current DBA assignment mechanism has a time delay, and massive burst data packet losses occur due to an insufficient buffer of the ONU. For example, a bandwidth grant size assigned by an OLT to a single T-CONT of a single ONU in advance is 50 M. When the ONU requests the OLT for 100 M grant bandwidth for the T-CONT because uplink burst traffic of the ONU within a grant bandwidth period is 100 M, the OLT determines that a maximum bandwidth grant size that is currently set for the T-CONT of the ONU is 50 M, and the OLT assigns 50 M bandwidth to the ONU within the grant bandwidth period. In addition, remaining 50 M is temporarily stored in the ONU, and is transmitted within a next grant bandwidth period. As a result, the ONU requires at least two grant bandwidth periods to complete transmission of the 100 M data. In addition, if a buffer of the ONU is too small to temporarily store the 50 M burst traffic, the remaining burst traffic is discarded, and performance of an entire service is reduced.